[FIX] website_event: allow to customize "Use this tag" sentence

As the <strong/> tag which surrounds the "Use this tag" sentence also
contained a <a/> whose href is a computed one (t-att-href), the strong
was not able to be edited.

addons/website_event/views/website_event_templates.xml

@@ -381,9 +381,10 @@
                             Find out what people see and say about this event,
                             and join the conversation.
                         </p>
-                        <p><strong>Use this tag:
+                        <p>
+                            <strong>Use this tag:</strong>
                             <a t-att-href="'http://twitter.com/search?q=%23'+event.twitter_hashtag" target="_blank" class="label label-primary">#<span t-field="event.twitter_hashtag"/></a>
-                        </strong></p>
+                        </p>
                     </div>
                 </div>
             </div>